This study will provide information concerning the metabolic fate of leucine ingested either as free amino acid in an amino acid mixture or in a protein bound form as intrinsiclly labeled casein. This will be done under conditions of a generous and a semi-limiting leucine intake. Additionally, by administering (2H2)-phenylalanine orally and (2H5)- phenylalanine intravenously this will enable us to compare the effects of feeding amino acids or protein together with the tracer under generous and semi-limiting protein or amino acid supply. Also, the comparison among the two routes of tracer administration will give us quantitative information about the splanchnic extraction of leucine and phenylalanine during different dietary conditions.